But a source close to Selvadurai tells us the Foursquare cofounder has seemed "frustrated" and "lost."

"Naveen is a proud guy and a real entrepreneur -- someone who probably wouldn't want to remain in a situation that wasn't fulfilling for him," says the source.

"It can't be easy to be the guy behind the scenes while Dennis is always out front as CEO."

Dennis Crowley has really shone as the leader of Foursquare, in part because he lives and breathes the app. An avid socializer, Crowley's colleagues say they've never seen a CEO who more deeply embodies his product. Crowley, who hasn't responded to our request for comment, says he's been trying to solve the same "where are my friends" problem for many years.